This relates generally to compositing and particularly to alpha blending in connection with graphics pipelines.
A graphics pipeline, typically executed by a graphics processing unit, performs a series of operations that result in the display of graphical images on computer displays. Compositing is a process of building the image, typically from back to front, but also possibly from front to back. Back to front compositing builds layer after layer, starting with the rearmost layer (i.e. the one farthest from the viewer). Each layer is composited on top of its predecessors. In front to back compositing, the foreground image plane is generated and then the layers that lie behind that image plane are successively produced.
Two-dimensional casual games and other applications very often perform compositing by drawing the image in a back to front order, always painting closer objects on top of objects further away. Each of the elements is drawn with an alpha value signifying whether a pixel is opaque, transparent, or somewhere between opaque and transparent. This process of repeatedly painting one object over the other results in significant amounts of bandwidth usage. Even simple casual games and other tasks, such as web page rendering, can consume significant amounts of power due to this process of repeatedly painting one object over the other.
In the case to front to back rendering, a rendered target needs to store an alpha channel, indicating how opaque each pixel is. Assuming an object's color is C and alpha value of N (between 0.0 for fully transparent, and 1.0 for fully opaque), and the render target's color is P and alpha value is M, the pixel may be updated as follows:P′=(1−M)*N*C+P. M′=M+(1−M)*N 
The set of rules about how to update the color and alpha value of a pixel is called the blend mode. The blend mode for front to back rendering is generally called destination alpha or DestAlpha blending in OpenGL and Direct3D.
The range of M may also be reversed in some cases, such that an alpha value of zero means fully opaque and a value of one means fully transparent.